The invention relates in general to pressure measurement and in particular to the measurement of fluid pressure on projectiles launched from gun tubes.
Projectiles launched from tubes such as barrels, rifles, cannons, etc. require a gas seal. The gas seal enables pressure to build up behind the projectile. The pressure applies a force on the base of the projectile thereby accelerating the projectile down the barrel and achieving a desired muzzle velocity. The desired muzzle velocity enables the projectile to fly far enough or fast enough to impact a point downrange.
Gas seals have evolved over the years from gas checks, paper, plastic, metal, wadding or a combination thereof to obturators. Obturators can be highly efficient pressure seals, but in most launch tubes some amount of propellant gas or pressure escapes past the obturator. The escaped gas or pressure is known as blow-by.
The obturator can be located almost anywhere on the projectile, for example, behind the projectile, on the projectile base or toward the front of the projectile. Regardless of the location of the obturator, some pressure will leak past the obturator and act on the projectile. The amount of leakage or blow-by is often not well characterized.
Pressure leakage past the obturator can be detrimental to the round or projectile. Blow-by may prevent the round from exiting the tube by imposing a buckling force on the projectile. Blow-by may mechanically deform the round and/or damage it so that the round interferes with the barrel wall. Interference of the projectile with the barrel wall may ruin the barrel and/or the projectile. In the case of high explosive rounds, a deformed projectile may explode and injure or kill nearby personnel.
Measurements of, for example, the pressure on the base of a gun-launched projectile, the amount of blow-by gas, and the pressure of blow-by gas are needed for many reasons. These measurements are needed, for example, to determine the interior ballistics of a weapon system, the margin of safety for the pressure seal or obturator, the characteristics of a new projectile, and/or the interoperability of munitions across standardized weapons.
Past methods of determining acceptable performance of gun-launched projectiles include statistical analysis of large numbers of fired rounds. The statistical methods enable one to gain confidence in a particular projectile design only by the sheer volume of successful firings. When the cost of projectiles is high or if a projectile is in a developmental stage, a more cost effective and scientific method is the use of instrumented projectiles. Instrumented projectiles can be used to evaluate interior ballistics, measure the amount of base pressure, and measure the amount of blow-by. The instruments may be pressure gauges or electronic instruments.
Pressure gauges are most often electro-mechanical systems that use piezoelectric materials. When crushed, the piezoelectric materials generate a voltage. The amount of voltage is proportional to the amount of force applied to the crystal or materials, so one can calibrate a gauge and correlate the measured voltage to a well-defined static or dynamic pressure. When placed on projectiles, these gauges experience extreme accelerations. These accelerations also apply inertial loads to the piezoelectric materials thereby imparting some bias or error to the measured signal. These gauges also require ports to enable the pressure to act on the gauge. The ports must be recessed enough to prevent impact from the propellant gas and from the barrel. The manner in which the ports are exposed to the pressure is often of significance because pressure waves within the port can prevent accurate measurements. Even if the gauge components are correctly machined, calibrated, oriented, ported and installed, the electronic data collecting system is very expensive and susceptible to damage. The damage leads to erroneous measurements.
Another method of measuring pressure on a projectile uses pressure-sensitive tape applied to the outside of the projectile. The tape materials are similar to the gauges in that the crystal structure of the tape has a known resonant frequency and wavelength. When the tape is applied to the projectile and exposed to the gas pressure, the change in the crystal structure produces a frequency shift that can be observed as a change in color. Pressure-sensitive tapes are, to some extent, sensitive to the heat applied to them by the combustion of the propellant. In addition, the pressure-sensitive tapes can separate from the projectile and scatter across the test range, thereby eliminating their usefulness. Even if the tape remains on the projectile, the change in material structure and appearance may require spectrum analysis because the color shift may be perceived differently by different observers.
One method of measuring gun chamber pressures uses copper spheres and anvils. When compressed, the anvil crushes the sphere and creates two flats on the sphere. The measurement across the flats is compared to previously validated data to determine maximum chamber pressure. The copper spheres are not suitable for use on projectiles.
Examples of pressure measurement methods are disclosed in U.S Patent Application Publication US2012/0312092 published on Dec. 13, 2012; U.S. Pat. No. 7,600,421 issued on Oct. 13, 2009; U.S. Pat. No. 7,058,549 issued on Jun. 6, 2006; U.S. Pat. No. 3,706,229 issued on Dec. 19, 1972; and U.S. Pat. No. 3,122,919 issued on Mar. 3, 1964.
A need exists for a low cost, accurate, and dependable apparatus and method for measuring fluid pressure on gun-launched projectiles.